1. Field of the Invention
The present invention relates in general to an exposure method using photomask shifting. In particular, the present invention relates to an exposure method to define two adjacent patterns in which adjacent and same patterns are transferred by a plurality of exposures with a single photomask. Therefore, the distorteded appearance of adjacent patterns due to proximity of transparent regions in a photomask is avoided.
2. Description of the Related Art
In semiconductor production, the process for transferring a photomask pattern to a resist material on a semiconductor wafer is referred to as the photolithographic process.
In recent years, along with the increasing miniaturization of the semiconductor devices produced, design rule has become smaller and lithography is being performed near the theoretical lower limit of resolution. This trend propagates the disadvantages of deterioration in performance of semiconductor devices due to deformation of the transfer pattern and reduction of yield due to bridging (error connection) and disconnection of the patterns.
FIG. 1 depicts transparent regions located on a photomask. The photomask 11 comprises a plurality of transparent regions (121-126). As shown in FIG. 1, the distance X between transparent regions 121 and 122 is much shorter than the distance Y between transparent regions 124 and 125. Therefore, the patterns transferred by the transparent regions 121 and 122 are distorted. Due to the image distortion, the original design may not be printed exactly.
The formation of photomask 11 is usually applied to an integrated circuit with regular design, such as DRAM circuits. To eliminate the optical error, better optical lens and process control are required. However, this is very difficult and costly.
The present invention provides an exposure method to define two adjacent patterns. In the present invention, adjacent and identical pattern groups are transferred by a plurality of exposures with a single photomask. Therefore, the distorteded appearance of adjacent patterns due to the proximity of transparent regions of a photomask is avoided. In addition, the elements of the pattern group are dependent upon the design pattern. Therefore, patterns with short distances will be printed exactly. In addition, the single photomask is used several times, so the cost of the process is not increased. Moreover, the process of exposure only entails shifting the photomask, and is thus a simple process.
To achieve the above-mentioned object, the present invention provides a shifting multi-exposure method for defining a regular pattern by a photomask. The method comprises the following steps. First, a photoresist layer comprising a first region and a second region is formed on a substrate. Then, a first pattern is defined on the first region by the photomask. Next, the photomask is moved for a predetermined distance, and then a second pattern is defined on the second region by the photomask. Finally, development is performed to display the first pattern and the second pattern on the photoresist layer.
Moreover, the present invention provides a shift multi-exposure method for defining a regular pattern by a photomask. The method comprises the following steps. First, a photoresist layer comprising a first region and a second region is formed on a substrate. Then, a pattern element is defined on the first region by the photomask. Next, the photomask is moved for a predetermined distance, and the pattern element is defined again on the second region by the photomask. Finally, development is performed to display the pattern elements on the photoresist layer.